"there is no way our small sub would do that."
Wanna bet?
Ding ding ding ding! I just found the source of your problem. Bass travels a LOT better than you think in an apartment building (particularly if it wasn't built with accoustical insulation in mind.)
This is very, very true. It travels far, and can do funny things as it's traveling. I'm of the mindset that essentially no one should have any respectably-sized subwoofer plugged in when living in a multi-floor apartment building, but that's just me. Regardless, if it's already led to a complaint, then your neighbors are probably going to be hypersensitive about any future noise. Your best bet is to unplug it entirely at this point.
As for the neighbors not talking with you, I agree it'd be easier if they did so, but you have to see it from their perspective as well--while it's possible it could go very well, it's also possible that as far as they know things could break bad, you'd get upset, and would then go out of your way to make their lives hell. In my own experience, I've spoken with people directly about unnecessary noise, and while they've been nice about it, it generally resulted in nothing changing until management got involved. From the other side, I've also lived with people who generally didn't much care about a neighbor asking them to quiet down; they'd respond nicely, but would then go right back to what they were doing. Thus, at this point, a large part of me favors the "go straight to the landlord route."
With all that being said, as others have suggested, just talk to your landlord. Find out more about the complaints, have receipts ready to show you were out of town, say what you've done to appropriately respond to the previous complaint, etc. The eviction process is a huge PITA from what I've been told, so I'm sure they'd much rather have you speak with them and try to work out a solution (even if it's some sort of discount on an early lease termination) than have to go to court.
Also, like another poster mentioned, as far as I'm aware, there's no credit-related penalty for breaking a lease early so long as you pay any associated fees. I did it myself the previous time I moved, as my then-future roommate needed me in by June while my lease didn't expire until August. My apartment complex was fine with it, gave me my security deposit back afterward, and even offered to serve as a renting reference in the future should I need it.
However, if your roommate is the one who's really pushing for breaking the lease, then I'd definitely say he should be responsible for most, if not all, of the early-termination fee (which is usually something like one or two months' rent).